This invention relates in general to a method of etching a shadow mask, for a color television picture tube, and in particular to a method of etching the steel substrate layer of a two layer shadow mask, wherein the first layer has been previously etched, and of reclaiming the used etchant.
Conventionally, blanks for shadow masks for color television picture tubes, typically composed of 5-6 mil steel, have been etched with a ferric chloride etchant. Areas of the blank which are not to be etched are protected by a patterned layer of suitable photoresist. As the etching takes place, the spent ferric chloride etchant is regenerated by adding chlorine to the used etchant. Another etchant which may be used is ferric sulphate, however, this etchant is not commercially desirable since it is very expensive and much slower in its chemical milling action than ferric chloride.
This invention has general applicability to the etching of color CTR shadow masks, but is most advantageously applied to the etching of a novel color television picture tube shadow mask, details of which are disclosed in U.S. Pat. No. 3,794,873. This shadow mask is composed of two layers of different metals - a thin aperture-defining layer, preferably nickel, disposed on a substrate layer, preferably steel. Initially, a ferric chloride etchant may be used to etch a pattern of holes in the aperture-defining layer (here assumed to be composed of nickel) through an exposed and developed photoresist layer. However, to further etch through the steel substrate layer, an etchant which will not attack the nickel, but which will etch the steel, is needed. An etchant of ferric sulphate would meet the requirements for this application, however, prior to this invention it would be too expensive to use.